U.S. patent number 4,258,375 [Application Number 06/028,319] was granted by the patent office on 1981-03-24 for ga.sub.x in.sub.1-x as.sub.y p.sub.1-y /inp avalanche photodiode and method for its fabrication.
This patent grant is currently assigned to Massachusetts Institute of Technology. Invention is credited to Jaw J. Hsieh, Charles E. Hurwitz.
United States Patent |
4,258,375 |
Hsieh , et al. |
March 24, 1981 |
Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y /InP Avalanche photodiode
and method for its fabrication
Abstract
An improved avalanche photodiode having an active layer of
Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y containing a p-n junction
and a window layer grown epitaxially to an n.sup.+ substrate is
disclosed herein, as well as methods for its fabrication.
Inventors: |
Hsieh; Jaw J. (Burlington,
MA), Hurwitz; Charles E. (Lexington, MA) |
Assignee: |
Massachusetts Institute of
Technology (Cambridge, MA)
|
Family
ID: |
21842782 |
Appl.
No.: |
06/028,319 |
Filed: |
April 9, 1979 |
Current U.S.
Class: |
257/186; 257/196;
257/201; 257/E31.063 |
Current CPC
Class: |
H01L
31/107 (20130101) |
Current International
Class: |
H01L
31/102 (20060101); H01L 31/107 (20060101); H01L
029/161 () |
Field of
Search: |
;357/30,13,17,61,52,16 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Hurwitz et al., "Topical Meetin on Integrated and Guided Wave
Optics" Digest of Technical Papers, Jan. 10-18, 1977, pp. mc1-1,2,
and 3. .
Hurwitz et al., "GaInAsP/InP Avalanche Photodiode", full paper
presented at Topical Meeting, Jan. 1977, Salt Lake City, UT. .
Hurwitz et al., Appl. Phys. Lett., 32(8), Apr. 15, 1978, pp.
487-489. .
Ito et al., Electronics Letters, 14, pp. 418-419, Jul. 6, 1978.
.
Lee et al., IEEE J. Quantum Electronics, vol. qe-15, No. 1, Jan.
1979, pp. 30-35. .
Takanashi, et al., Jap. J. Appl. Phys., 17(1978), No. 11, pp.
2065-2066. .
Weider et al., Appl. Phys. Lett., 31, No. 7, Oct. 1, 1977, pp.
468-470. .
Clawson et al., Appl. Phys. Letts., 32(9), May 1, 1978, pp.
549-551..
|
Primary Examiner: Edlow; Martin H.
Attorney, Agent or Firm: Smith, Jr.; Arthur A. Brook; David
E.
Government Interests
GOVERNMENT SUPPORT
Work relating to this invention was supported by the United States
Air Force.
Claims
We claim:
1. An avalanche photodiode, comprising:
a. an n+ InP substrate;
b. an active layer of Ga.sub.x In.sub.1-x As.sub.y P.sub.1--y
lattice-matched to said substrate and having a p-n junction
sufficient to produce an avalanche effect in said photodiode;
c. a window layer lattice-matched to said substrate, said window
layer having a bandgap equal to or higher than the bandgap of said
active layer and being formed from InP or a quaternary having the
formula Ga.sub.x 'In.sub.1-x 'As.sub.y 'P.sub.1-y ' wherein
0.ltoreq.x'.ltoreq.x and 0.ltoreq.y'.ltoreq.y; and,
d. ohmic contacts to said avalanche photodiode.
2. An avalanche photodiode of claim 1 wherein said window layer is
formed from Ga.sub.x 'In.sub.1-x 'As.sub.y 'P.sub.1-y, wherein
0.ltoreq.x'.ltoreq.x and 0.ltoreq.y'.ltoreq.y.
3. An avalanche photodiode of claim 2 wherein said substrate has a
substantially (111)B of (100) orientation.
4. An avalanche photodiode of claim 3 wherein said active layer of
Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y contains a p-n
homojunction.
5. The method of fabricating an avalanche photodiode,
comprising:
a. depositing an epitaxial lattice-matched active layer of n-type
Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y on an n+-type InP
substrate;
b. depositing an epitaxial lattice-matched window layer of Ga.sub.x
'In.sub.1-x 'As.sub.y 'P.sub.1-y ', wherein 0.ltoreq.x'.ltoreq.x
and 0.ltoreq.y'.ltoreq.y, over said Ga.sub.x In.sub.1-x As.sub.y
P.sub.1-y layer;
c. diffusing or implanting p-type dopant into said window layer of
Ga.sub.x 'In.sub.1-x 'As.sub.y 'P.sub.1-y ' and said active layer
of Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y to form a p-n junction
therein; and,
d. applying ohmic contacts to said substrate and said window layer
of said avalanche photodiode.
6. The method of claim 5 wherein said p-n junction is 0 to about
1.5 .mu.m from the interface between said active and said window
layers.
7. A method of claim 6 wherein said avalanche photodiode is etched
to provide a mesa structure.
8. A method of claim 6 wherein a guard ring structure is formed in
the n-type Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y active layer by
diffusing therein p-type dopant.
9. A method of claim 6 wherein a guard ring structure is formed in
said n-type Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y active layer by
implanting therein p-type ions.
10. A method of claim 6 wherein the surface of said n.sup.+-type
InP substrate is melted back prior to deposition of said layer of
n-type Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y.
11. A method of claim 6 wherein a buffer layer of InP is deposited
upon said n.sup.+ -type InP substrate prior to depositing said
n-type Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y active layer
thereon.
12. A method of claim 6 wherein said p-type dopant comprises zinc
atoms.
Description
TECHNICAL FIELD
This invention is in the field of solid state devices and more
particularly relates to avalanche photodiodes formed from
GaInAsP.
BACKGROUND ART
Double-heterostructure diode lasers based upon epitaxial layers of
GaInAsP grown upon InP substrates have been disclosed. See Bogatov,
A. P., Dolginov, L. M., Druzhinina, L. V., Eliseev, P. G.,
Sverdlov, B. N., and Schevchenko, E. G., Sov. J. Quantum Electron.,
4, 1281 (1975); Hsieh, J. J., Appl. Phys. Lett., 28, 283 (1976);
and Pearsall, T. P., Miller, B. I., Capik, R. J. and Bachman, K.
J., Appl. Phys. Lett., 28, 499 (1976). The emission wavelength of
such lasers can be controlled within the range of about 0.95-1.70
.mu.m at room temperature without significantly detracting from
good lattice-matching by simply changing the composition of the
quaternary solid solution of GaInAsP. Since this range includes
1.1-1.3 .mu.m, the region currently thought to be optimum for
optical communication systems utilizing fused silica fibers, these
lasers are prime candidates for use in optical communications.
In order for such lasers to be utilized in optical communication
systems, nevertheless, detector systems capable of operating within
this wavelength range will also be necessary. In particular, it
would be advantageous to have an avalanche photodiode capable of
operating within this wavelength range.
Photodiodes fabricated from silicon and germanium have been known
for many years. Silicon, however, has a low quantum efficiency for
wavelengths longer than 1.1 .mu.m. Germanium, on the other hand,
has nearly equal ionization coefficients for electrons and holes,
which leads to a large excess noise factor and inherently lower
speed of response. Germanium also has a smaller than optimum
bandgap for 1.3 .mu.m peak wavelength response which results in a
large dark current unless cooled below room temperature.
More recently, avalanche photodiodes have been fabricated from
materials including GaInAs and GaAsSb epitaxially grown on GaAs
substrates. In these systems, severe lattice-mismatches between the
epitaxial layer and the GaAs substrate have necessitated
intermediate matching layers of other compositions. This has
necessitated complicated and delicate structures which require
elaborate fabrication procedures. Additionally, yields of
reasonably performing devices have been very low. Furthermore, most
of the work done in regard to detectors with these systems have
resulted in detectors which respond only out to wavelengths of
about 1.1 .mu.m. The lattice-mismatch problems become much more
severe for compositions capable of responding to longer
wavelengths.
Avalanche photodiodes have also been fabricated from the ternary
alloy GaInAs grown lattice-matched to InP substrates. See Pearsall,
T. P. and Hopson, Jr., R. W., J. Electron. Mater., 7, 133 (1978).
Although avalanche gain was noted at 1.2 .mu.m, the peak response
with this system was fixed at about 1.6 .mu.m and could not to be
chosen to optically respond to any particular desired wavelength.
Further, the single heterostructure design led to the loss of
photogenerated carriers by surface recombination, leading to
reduced quantum efficiency.
Even more recently, avalanche photodiodes have been fabricated
employing the quaternary GaInAsP on InP substrates. See Hurwitz, C.
E. and Hsieh, J. J., "Topical Meeting on Integrated and Guidewave
Optics", Digest of Technical Papers presented Jan. 16-18, 1977,
pages MCl-1, 2 and 3 (1978).
DISCLOSURE OF THE INVENTION
This invention relates to improved avalanche photodiodes based upon
quaternary layers of Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y. The
avalanche photodiodes of this invention are formed from an n+ InP
substrate upon which an active layer of Ga.sub.x In.sub.1-x
As.sub.y P.sub.1-y is epitaxially deposited to achieve a good
lattice match followed by deposition of a top window layer. A p-n
junction is formed and ohmic contacts are applied to the front and
back surfaces of the device. The p-n junction contained within the
Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y active layer can be formed
by diffusing a p-dopant, such as zinc atoms, into the window and
Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y layers from the top of the
device.
This structure has significant advantages over avalanche
photodiodes based upon Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y
active layers deposited on p substrates. For example, an avalanche
photodiode formed on an n-type substrate should have less noise
than the corresponding photodiode formed on a p-type substrate
under top illumination. This is true because the electron
ionization rate of Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y is higher
than that of holes. It is possible, of course, to detect
illumination from the substrate side of a photodiode grown on a
p-type substrate to thereby obtain a similar noise value
corresponding to that obtained with a photodiode grown on a n-type
substrate and sensed from the window layer side, but this
necessitates more complicated procedures in fabricating such a
photodiode and the device is mechanically weaker when the device is
mounted on a package from its window layer side, particularly when
the device has a mesa structure. In the present invention, the
improved device permits diffusion of p-type dopants to be performed
from the mesa window and also allows the p-type layers to be formed
by other techniques such as ion implantation. The capability to
diffuse p-type dopants from the top window layer is important to
the ultimate device performance because it allows control of
p-dopant concentration as well as diffusion depth in a much more
accurate manner.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of an avalanche photodiode
according to this invention; and,
FIG. 2 is a cross-sectional view of an alternative embodiment of an
avalanche photodiode according to this invention in which a guard
ring structure has been added.
BEST MODE OF CARRYING OUT THE INVENTION
The invention can be further described with particular reference to
the Figures.
An avalanche photodiode 10 according to this invention is
illustrated in FIG. 1. Avalanche photodiode 10 is formed by
starting with n+-InP substrate 12. Substrate 12 preferably has a
substantially (111)B or (100) orientation, although exact
orientation is not critical. Substrate 12 might comprise, for
example, a Czochralaski-grown crystal of InP doped with Sn or other
n-type dopants to a carrier concentration of about 10.sup.18
carrier/cm.sup.3. The substrate surface can be lapped with 2 .mu.m
grit of alumina, chemi-mechanically polished with Br--CH.sub.3 OH,
and then free-etched with Br--CH.sub.3 OH to remove an additional
10 .mu.m of material. It is possible, of course, to grow active
layers of Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y on InP substrates
having other orientations, and with other techniques of surface
preparation.
Subsequently, n-type Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y layer
14 is grown on InP substrate 12 using a technique such as
super-cooled liquid phase epitaxy (LPE). A thorough discussion of
the growth of liquid phase epitaxy layers by the super-cooling,
step-cooling, equilibrium-cooling and 2-phase solution techniques
is presented in Hsieh, J. J., "Thickness and Surface Morphology of
GaAs LEP Layers Grown by Super-Cooling, Step-Cooling,
Equilibrium-Cooling and 2-phase Solution Techniques," J. Cryst.
Growth, 27, 49-61 (1974). LPE techniques from super-cooled
solutions are also described for the growth of Ga.sub.x In.sub.1-x
As.sub.y P.sub.1-y layers on InP substrates for the purpose of
fabricating double-heterostructure lasers in copending application
Ser. No. 816,402, filed July 18, 1977.
Although super-cooling liquid-phase epitaxy is the preferred
method, other growth techniques, including step-cooling,
equilibrium-cooling and 2-phase solution techniques are also
suitable.
The thickness of active Ga.sub.x In.sub.1-x As P.sub.1-y layer 14
can vary and a typical range would be 2-5 .mu.m. Window layer 16 is
epitaxially deposited on Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y
layer 14. Window layer 16 can be indium phosphide or a quaternary
having the formula Ga.sub.x' In.sub.1-x,As.sub.y',P.sub.1-y'
wherein 0.ltoreq.x'.ltoreq.x and 0.ltoreq.y'.ltoreq.y. Window layer
16 has a bandgap which is equal to or higher than the bandgap of
Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y layer 14. Window layer 16
can be either n or p-type and a typical thickness for window layer
16 would be about one .mu.m.
Subsequently, p-dopant can be diffused into window layer 16 and
n-type Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y layer 14 to produce
p-type Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y layer 14'. This can
be done, for example, by sealing the device in an ampoule
containing a source of zinc atoms, such as ZnP.sub.2. A typical
diffusion would be carried out at 500.degree. C. for about an hour.
Such diffusion typically results in the formation of a p-n
homojunction located a distance of up to about 1.5 .mu.m from the
interface between window layer 16 and p-type layer 14'.
Alternately, p-type layer 14' could be formed by ion implantation,
or other suitable techniques. In some cases, it would even be
suitable to omit diffusion of p-type dopants or ion implantation
and simply to rely upon the heterojunction formed between a p-type
window layer 16 and n-type active layer 14.
Ohmic contacts are then applied to both sides of the device. Ohmic
contact 20 can be formed by evaporating a layer of Au--Sn whereas
ohmic contact 22 can be formed by evaporating a thin layer of
Au--Mg and then defining a small area using standard
photo-lithographic techniques, each followed by alloying at
400.degree. C. in a nitrogen atmosphere. Following application of
ohmic contacts 20 and 22, the device is etched using a solution
such as 1% Br in methanol to provide the mesa structure illustrated
in FIG. 1. The purpose of forming the desired mesa structure is to
minimize or prevent edge breakdown of the devices, as is known in
the art.
Other methods of preventing edge breakdown can be employed, of
course. One such method is illustrated in the device shown in FIG.
2 in which the diffused or implanted p layer 26 is surrounded by a
guard ring structure 28. Guard ring structure 28 is formed from a p
region which can be provided also by diffusion or ion implantation.
Suitable guard ring structures are known in the art. See, for
example, Anderson, L. K. and McMurtry, B. J., "High-speed
Photodetectors", Proceedings of the IEEE, 54, No. 10, Oct. 1966. In
this device, the top ohmic contacts 30 are not deposited
continuously across the top surface. Ohmic contacts 30 can be
formed, however, from the same material as ohmic contact 22 in FIG.
1. Other similar layers and materials in FIG. 2 are referred to by
the same numerals as were previously employed in describing FIG.
1.
In growing the active layer of Ga.sub.x In.sub.1-x As.sub.y
P.sub.1-y upon the n+ InP substrate, it is preferred to employ a
melt back technique and/or the use of a thin buffer layer of InP.
The former is described by V. Wrick et al. at Electr. Lettrs, 12,
pp 394-5 (1976), whereas the latter is described in copending
application Ser. No. 816,402, filed July 18, 1977.
INDUSTRIAL APPLICABILITY
This invention has industrial applicability in the fabrication of
avalanche photodiode detectors which are particularly suitable in
the 1.0-1.6 wavelength region.
EQUIVALENTS
Those skilled in the art will recognize, or will be able to
ascertain using no more than routine experimentation, many
equivalents to the specific embodiments of the invention described
herein. Such equivalents are intended to be encompassed by the
following claims.
* * * * *